Many microwave and RF systems use couplers in order to separate transmit (TX) and receive (RX) signals or to sum two combined signals. Systems having couplers used in this dual role may be referred to as transmit-receive (TRX) systems. For example, such TRX systems are often used in automotive radar systems utilizing frequency modulated continuous wave (FMCW) radar.
These TRX systems require good isolation between the high-power transmit signal and the often very weak receive signals. Inherently, a coupler used as a duplexer with isolation has to have losses in order to be realizable according to traditional network theory. In this case, only half of the incoming power is fed to the output of the duplexer, while the other half is dissipated in a suitable termination.